The present disclosure relates generally to Light-Emitting Diode (LED) lamps, and more particularly to Alternating Current (AC) driven LED lamps and control methods thereof.
Light-Emitting Diodes or LEDs are increasingly being used for general lighting purposes. In one example, a set of LEDs is powered from an AC power source and the term “AC LED” is sometimes used to refer to such circuit. Concerns for AC LED include manufacture cost, power efficiency, power factor, flicker, lifespan, etc.
FIG. 1 demonstrates AC LED lamp 10 in the art, which, in view of electric circuit, simply has a LED module 12 and a current-limiting resistor 14. The LED module 12 consists of two LED strings connected in anti-parallel. The AC LED circuit 10 in FIG. 1 requires neither an AC-DC converter nor a rectifier. Even though a DC voltage is also compatible, an AC voltage is typically supplied to input port 8 and directly powers the AC LED circuit 10. Simplicity in structure and low-price in manufacture are two advantages the AC LED circuit 10 provides. Nevertheless, the AC LED circuit 10 can only emit light in a very narrow time period in each AC cycle time, suffering either low average luminance or high-current stress to LEDs.
FIG. 2 demonstrates another AC LED lamp 15 in the art. Examples of the AC LED lamp 15 can be found from U.S. Pat. No. 7,708,172. The AC LED lamp 15 employs full-wave rectifier 18. A DC or AC voltage source is received on input port 16. A string of LEDs are grouped into LED groups 201, 202, 203, and 204. An integrated circuit 22 has nodes PIN1, PIN2, PIN3, and PIN4, connected to the cathodes of LED groups 201, 202, 203, and 204 respectively. Inside integrated circuit 22 are path switches SG1, SG2, SG3, and SG4, and a path controller 24 as well. When the voltage on input port 16 increases, controller 24 can switch path switches SG1, SG2, SG3, and SG4, to possibly make more LEDs emit light. Operations of integrated circuit 22 have been exemplified in U.S. Pat. No. 7,708,172 and are omitted here for brevity.
FIG. 3 illustrates the waveforms of signals when the input port 16 in FIG. 2 is supplied with an AC sinusoidal input voltage. The upmost waveform shows a rectified input voltage VREC, which, as indicated in FIG. 2, refers to the voltage the full-wave rectifier 18 provides to LED group 201. The second waveform shows the total number of illuminating LEDs, meaning the number of LEDs that are illuminating. The four following waveforms regard with currents IG4, IG3, IG2 and IG1, which as shown in FIG. 2, refer to the currents flowing through LED groups 204, 203, 202 and 201, respectively. The total number of illuminating LEDs rises or descends stepwise, following the increase or decrease of the rectified input voltage VREC. When the rectified input voltage VREC increases, LED groups 201, 202, 203, and 204, according to a forward sequence, join to illuminate. For example, when the rectified input voltage VREC increases to just exceed the threshold voltage VTH1, the voltage required for driving the LED group 201 to illuminate, the LED group 201 starts illuminating. When the rectified voltage VREC decreases, LED groups 201, 202, 203, and 204 darken according to a backward sequence. If, for example, the rectified input voltage VREC just falls below the threshold voltage VTH4, the voltage required for driving all the LED groups 201, 202, 203 and 204 to illuminate, then the path switch SG3 is switched ON, such that the LED group 204 stops illuminating, leaving only the LED groups 201, 202 and 203 to emit light. The AC LED lamp 15 enjoys simple circuit architecture and, as can be derived, good power efficiency.
There in FIG. 3 however has a dark period TDARK when no LED illuminate, because the rectified voltage VREC is too low to drive the LED group 201. If the rectified voltage VREC is a 120-Hertz signal, the voltage valley, where the rectified voltage VREC is about zero Volt, appears at 120 Hz, causing the dark period TDARK to show up in the same frequency of 120 Hz. Even though the dark period TDARK at 120 Hertz might not be perceivable by human eyes, reports indicate that when a digital camera takes a photograph of an object exposed under the luminance of the LED lamp 15, the photograph comes out with unwelcome dark lines due to the existence of the dark period TDARK. It is appreciated for an LED lamp to introduce no dark period TDARK.